Start by cutting power to the furnace or air conditioner at the breaker so you can safely test the blower motor capacitor.

With power off, remove the access panel and locate the run capacitor mounted near the blower motor. Discharge the capacitor by shorting its terminals with a insulated screwdriver that has had the insulation removed from the shaft end or by using a shorting tool; this removes any stored charge so you will not get shocked or damage your meter. Carefully disconnect the capacitor leads, noting or labeling each terminal so you can reconnect correctly. Set a digital multimeter to the capacitance setting and touch the meter leads to the capacitor terminals; a three terminal capacitor will be marked with fan, hermetic, and common so test between common and fan for the blower circuit. Compare the reading to the printed microfarad rating on the capacitor; the measured value should be within plus or minus five percent of the rated value for proper operation. If the capacitor reads zero, is below the tolerance range, or shows signs of bulging, cracking, or oil leakage replace it with the exact microfarad and voltage rated unit and ensure correct terminal wiring.

A common mistake is forgetting to remove power before touching the capacitor which can cause a severe shock; always turn the breaker off and verify no voltage with a multimeter. Another frequent issue is not discharging the capacitor first which can give you an incorrect reading or injure you, so always short the terminals briefly after isolation. Many people replace the capacitor without matching the microfarad rating or using the correct voltage rating which leads to rapid failure, so always install a capacitor with the same microfarad and equal or higher voltage rating and confirm terminal wiring matches the original.

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